The present invention relates to a compatible picture telephone system for the digital transmission of narrowband video signals from a narrowband system and broadband signals from a broadband system, narrowband or broadband picture recording and playback instruments being selectively connected thereto depending on the desired quality of picture reproduction.
The Bell Laboratories has developed a videophone, or "Picturephone," the latter being a trademark owned by the Bell System, which is a narrowband picture telephone system which has a satisfactory picture resolution for the display of the image of a person's face. This system operates with a 267-line picture and a line scanning frequency of 8 KHz and requires a transmission bandwidth of 1 MHz. Such a system is described in the Bell Technical Journal, Volume 50, 1971, at pages 235-269.
If this system is to be used for data or information transmission purposes, however, such picture resolution is not sufficient. For example, the known system is able to transmit legibly only about one eighth of the area of a typewritten page of the size specified by DIN A4.
Picture telephone systems have been proposed which operate with a greater number of lines, a higher line scanning frequency and a greater transmission bandwidth and which produce a resolution corresponding to up to one-half of a DIN A4 typewritten page. Such a system is described in IEEE SPECTRUM, No.1973, at pages 45-49. Nov.
There thus exists the drawback that a number of independent picture telephone systems coexist which are not compatible with one another. Without the intermediary of complicated instruments, such as line converters, for example, it is impossible to effect picture telephone conversations between users employing such different picture telephone systems.